EP2124311A1

EP2124311A1 - Time delay compensation in power system control

Info

Publication number: EP2124311A1
Application number: EP08156785A
Authority: EP
Inventors: Petr Korba; Rafael Segundo; Bertil Berggren; Andrew Paice; Rajat Majumder
Original assignee: ABB Research Ltd Switzerland; ABB Research Ltd Sweden
Current assignee: ABB Research Ltd Switzerland; ABB Research Ltd Sweden
Priority date: 2008-05-23
Filing date: 2008-05-23
Publication date: 2009-11-25
Also published as: US8497602B2; US20110126038A1; CN102099982B; WO2009141297A1; EP2297832A1; CN102099982A; EP2297832B1

Abstract

The present invention is concerned with the compensation of time delays in remote feedback signals in power system control. A method is described which comprises converting the time delay into a phase shift and calculating four compensation angles from the phase shift. The optimal compensation angle is determined and applied to the remote feedback signals. A technique of equipping a controller with a global clock is also disclosed.

Description

FIELD OF THE INVENTION

The invention relates to the field of control of controllable devices in electric power systems interconnecting a plurality of generators and consumers. In particular, control of multimode electromechanical oscillations is considered, in which the controller utilises feedback signals.

BACKGROUND OF THE INVENTION

In the wake of the ongoing deregulations of the electric power markets, load transmission and wheeling of power from distant generators to local consumers has become common practice. As a consequence of the competition between power producing companies and the emerging need to optimize assets, increased amounts of electric power are transmitted through the existing networks, frequently causing congestions due to transmission bottlenecks. Transmission bottlenecks are typically handled by introducing transfer limits on transmission interfaces. This improves system security however it also implies that more costly power production has to be connected while less costly production is disconnected from the grid. Thus, transmission bottlenecks have a substantial cost to the society. If transfer limits are not respected, system security is degraded which may imply disconnection of a large number of customers or even complete blackouts in the event of credible contingencies.
The underlying physical cause of transmission bottlenecks is often related to the dynamics of the power system. A number of dynamic phenomena need to be avoided in order to certify sufficiently secure system operation, such as loss of synchronism, voltage collapse and growing electromechanical oscillations. In this regard, electrical transmission networks are highly dynamic and employ control systems and feedback to improve performance and increase transfer limits.
With particular reference to unwanted electromechanical oscillations that occur in parts of the power network, they generally have a frequency of less than a few Hz and are considered acceptable as long as they decay fast enough. They are initiated by e.g. normal changes in the system load or switching events in the network possibly following faults, and they are a characteristic of any power system. However, insufficiently damped oscillations may occur when the operating point of the power system is changed, for example, due to a new distribution of power flows following a connection or disconnection of generators, loads and/or transmission lines. In these cases, an increase in the transmitted power of a few MW may make the difference between stable oscillations and unstable oscillations which have the potential to cause a system collapse or result in loss of synchronism, loss of interconnections and ultimately the inability to supply electric power to the customer. Appropriate monitoring and control of the power system can help a network operator to accurately assess power system states and avoid a total blackout by taking appropriate actions such as the connection of specially designed oscillation damping equipment.
It has been found that electromechanical oscillations in electric power networks also take the form of a superposition of multiple oscillatory modes. These multiple oscillatory modes create similar problems to the single mode oscillations and thus have the potential to cause a collapse of the electric power network. Furthermore, in situations where a Power Oscillation Damping (POD) controller is used to stabilize a single selected oscillatory mode, this may often have the effect of destabilizing the other oscillatory modes present, for example, a second dominant mode, which is subsequently damped less than the first dominant mode.
Generally, power networks utilise so-called lead-lag controllers to improve undesirable frequency responses. Such a controller functions either as a lead controller or a lag controller at any given time point. In both cases a pole-zero pair is introduced into an open loop transfer function. The transfer function can be written in the Laplace domain as: $\frac{Y}{X} = \frac{s - z}{s - p}$

where X is the input to the compensator, Y is the output, s is the complex Laplace transform variable, z is the zero frequency and p is the pole frequency. The pole and zero are both typically negative. In a lead controller, the pole is left of the zero in the Argand plane, | z | < | p | , while in a lag controller | z | > | p | . A lead-lag controller consists of a lead controller cascaded with a lag controller. The overall transfer function can be written as: $\frac{Y}{X} = \frac{(s - z_{1}) (s - z_{2})}{(s - p_{1}) (s - p_{2})}$
Topically | p ₁ | > | z ₁ | > | z ₂ | > | p ₂ | , where z ₁ and p ₁ are the zero and pole of the lead controller and z ₂ and p ₂ are the zero and pole of the lag controller. The lead controller provides phase lead at high frequencies. This shifts the poles to the left, which enhances the responsiveness and stability of the system. The lag controller provides phase lag at low frequencies which reduces the steady state error.
The precise locations of the poles and zeros depend on both the desired characteristics of the closed loop response and the characteristics of the system being controlled. However, the pole and zero of the lag controller should be close together so as not to cause the poles to shift right, which could cause instability or slow convergence. Since their purpose is to affect the low frequency behaviour, they should be near the origin.
Electric power transmission and distribution systems or networks comprise high-voltage tie lines for connecting geographically separated regions, medium-voltage lines, and substations for transforming voltages and for switching connections between lines. For managing the network, it is known in the art to utilize Phasor Measurement Units (PMU). PMUs provide time-stamped local information about the network, in particular currents, voltages and load flows. A plurality of phasor measurements collected throughout the network by PMUs and processed at a central data processor, provide a snapshot of the overall electrical state of the power system.
The article "Application of FACTS Devices for Damping of Power System Oscillations", by R. Sadikovic et al., proceedings of the Power Tech conference 2005, June 27-30, St. Petersburg RU, the disclosure of which is incorporated herein for all purposes by way of reference. The article addresses the selection of the proper feedback signals and the subsequent adaptive tuning of the parameters of a power oscillation damping (POD) controller in case of changing operating conditions. It is based on a linearized system model, the transfer function G(s) of which is being expanded into a sum of N residues: $G (s) = \sum_{i = 1}^{N} \frac{R_{i}}{(s - λ_{i})}$
The N eigenvalues λ_i correspond to the N oscillation modes of the system, whereas the residue R_i for a particular mode gives the sensitivity of that mode's eigenvalue to feedback between the output and the input of the system. It should be noted that in complex analysis, the "residue" is a complex number which describes the behavior of line integrals of a meromorphic function around a singularity. Residues may be used to compute real integrals as well and allow the determination of more complicated path integrals via the residue theorem. Each residue represents a product of modal observability and controllability. Figure 1 provides a graphical illustration of the phase compensation angle φ_c in the s-plane caused by the POD controller in order to achieve the desired shift λ_k = α_k +j.ω_k of the selected/critical mode k, where α_k is the modal damping and ω_k is the modal frequency. The resulting phase compensation angle φ_c is obtained as the complement to +π and -π, respectively, for the sum of all partial angle contributions obtained at the frequency ω_k starting from the complex residue for mode λ_k, input I and output j, is Res_ji(λ_k), all employed (low- and high-pass) prefilters. φ_R is the angle of residue and φ_F is the phase shift caused by the prefilters.
Thus, it is known to utilise local feedback signals in power network control systems.
However, it is considered that power network control systems based on remote feedback signals may lead to substantial improvements in terms of damping unwanted electromechanical oscillations. However, there is a disadvantage associated with remote feedback signals. Specifically, remote signals are acquired by PMUs at distant geographical locations and sent via communication channels, which are potentially several thousand kilometres in length, to the controller. The remote signals may also pass through a wide-area data concentrating platform. Consequently, permanent time delays in the feedback loop result. It is known that such time delays may destabilize the feedback loop.
Figure 2 illustrates from where different time delays may originate. The PMUs, PMU1 to PMUn, each one being equipped with GPS synchronized clocks, will send positive sequence phasors at equidistant points in time, e.g. every 20 ms. These phasors are thus time stamped with high accuracy, and the time stamp may represent the point in time when it was measured in the system. However, the PMU has a processing time in order to establish the phasor, typically involving a Fourier analysis, which will imply a time delay Δt ₁. The communication channels between the PMUs and the Phasor Data Concentrator, PDC 10, may imply a time delay Δt ₂. The PDC, which synchronizes the phasors, i.e. packages the phasors with the same time stamp and sends them on to the control system 12, may imply a time delay Δt ₃. Although the phasors were time stamped with the same time stamp at the PMUs, they may arrive at the PDC 10 at slightly different points in time.
Since the PDC has to wait for the last phasor before it can send the data on, the package with synchronized phasors cannot have a smaller time delay than the largest time delay for any individual phasor. The communication between the PDC 10 and the control system 12 may imply a time delay Δt ₄. Obviously, if the PDC is integrated with the control system, this time delay can be omitted. The total time delay between the point in time when the remote measurement was performed and the point in time when the measurement is available in the control system may be of stochastic nature, i.e. may be varying around an expected value according to some distribution. This may reduce the performance of the control system, and may potentially be more detrimental than having no control signal.

DESCRIPTION OF THE INVENTION

It is therefore an objective of the invention to enable use of remote feedback signals to improve performance in electric power transmission networks in a flexible manner and with minimal additional complexity. These objectives are achieved by a method and a controller for compensation of a time delay in remote feedback signals in power system control according to claims 1 and 6, and an electric power control system according to claim 8. Further preferred embodiments are evident from the dependent claims.
According to a first aspect of the invention, a method is provided for compensation of a time delay in remote feedback signals in power system control. The method comprising determining a time delay of the remote feedback signals, converting the time delay into a phase shift, calculating four compensation angles from the phase shift, constructing a Nyquist diagram of each compensation angle, determining a preferred compensation angle through analysis of the four Nyquist diagrams, and applying the preferred compensation angle to the remote feedback signals. Advantageously, no additional hardware is necessary to implement this aspect of the invention, only parameters of an existing controller are modified.
Preferably, the method further comprises constructing a Bode diagram of at least two of the four compensation angles, and determining a preferred compensation angle through analysis of the Bode diagrams.
Further, the step of determining a preferred compensation angle through analysis of the Bode diagrams further comprises evaluating decay of controller's gain at higher and/or lower frequencies, respectively.
Further, the method further comprises constructing a complex frequency domain diagram of at least two of the four compensation angles, and determining a preferred compensation angle through analysis of the complex frequency domain diagram.
Also, the step of determining a preferred compensation angle through analysis of the complex frequency domain diagrams further comprises evaluating eigenvalue shift with respect to other system eigenvalues.
Preferably, the step of converting the time delay into a phase shift occurs at the dominant frequency.
In the method of the present invention, the four compensation angles are a lead compensation to +1 and a lag compensation to -1 and a lead compensation to -1 and a lag compensation to +1 of the phase shift signal.
According to a second aspect of the invention, a controller for a power system is provided for performing the method for compensation of a time delay in remote feedback signals of the first aspect of the invention. Further, a global clock arrangement for synchronising the controller with predetermined phasor measurement units is disclosed; the global clock arranged such that the time delay is determined on a continuous basis.
The controller of the present invention may be implemented as software run on a digital computer, or as a hard-wired implementation using techniques such as EPROM etc.
According to a third aspect of the invention, an electric power control system is provided. The system comprising at least one phasor measurement unit (PMU) equipped with transceiving means, a controller equipped with transceiving means, and a global clock for synchronising the controller with the at least one phasor measurement unit utilising the transceiving means, wherein the global clock is arranged such that a time delay of the system is determined on a continuous basis.
Preferably, the time delay of the system is estimated and the estimated time delay is utilised for phase compensation. Further, at least one parameter of the controller is adapted on-line based on the estimated time delay.
Alternatively, a running average of the time delay may be determined in the controller and the running mean value utilised for phase compensation.
Further, the PMU based control loop may be inactivated if the time delay is too large for the purpose of the control.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments which are illustrated in the attached drawings, of which:

Figure 1 graphically illustrates a theoretical pole-shift in the complex frequency domain of a known POD controller.
Figure 2 illustrates where different contributions of the total time delay may originate from.
Figure 3a graphically illustrates a pole-shift in the complex frequency domain of a POD controller in accordance with the present invention.
Figure 3b graphically illustrates the delayed measured signal and four possible solutions (A, B, C and D) for the compensation of the time delay in accordance with the present invention.
Figures 4a - 4d show Nyquist diagrams of the four possible solutions.
Figures 5a - 5d show Bode diagrams of the four possible solutions.
Figure 6 is a flow diagram of the operational method of the POD controller design of the present invention.
Figure 7 schematically illustrates a power system control environment integrating a global clock.
Figure 8 shows a block diagram of a power control system integrating a global clock.
The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Time delays in a feedback loop of a power network controller cannot be removed.
Therefore, the present invention compensates for the delays. Efficiently, known controllers can be used without the need to modify their structure. In order to compensate for the time delays, controller parameters are suitably adjusted in accordance with the present invention.
Figure 3a graphically illustrates a pole-shift in the s-plane for a POD controller in order to achieve the desired shift λ_k = α_k +j.ω_k of the mode of interest, k, where α_k is the modal damping and ω_k is the modal frequency. The resulting phase compensation angle φ_c is obtained as the complement to +π and -π, respectively, for the sum of all partial angle contributions obtained at the frequency ω_k starting from the complex residue for mode λ_k, input i and output j, is Res_ji(λ_k), all employed (low- and high-pass) prefilters. φ_R is the angle of residue and φ_F is the phase shift caused by the prefilters. φ_Td is the phase shift representing time delay Td at frequency ω_k.
The adjustment of the controller parameters is determined in the following manner. With reference to Figure 3b, a feedback signal is denoted by the dotted oscillating line. For simplicity, an undamped sine wave is shown. The feedback signal is phase shifted from the oscillating signal, represented by a solid line. The phase shift between the signal and the feedback signal is (ω_k.Td) where ω_k is the frequency of the mode being damped and Td is the time delay. Therefore, the time delay may be described as a phase shift at the oscillatory frequency of interest. It can be seen in Figure 3b that the time delay corresponds to lagging 60° at the dominant frequency ω. The related modified compensation angles are calculated from the residue, phi. In this example, phi is 80°. The four solutions for the modified compensation angle which compensate for the phase shift are described as; lag to +1, lag to -1, lead to +1, lead to -1. With reference to Figure 3b, the four solutions are graphically illustrated by the four points on the waves denoted as A, B, C, D, respectively. The actual values in this example can be seen to be -280°, -100°, 80°, 260°, respectively.
The next step in the adjustment of the controller parameters of the present invention utilises Nyquist diagrams. A Nyquist diagram is used in automatic control and signal processing for assessing the stability of a system with feedback. It is represented by a graph in which the gain and phase of a frequency response are plotted. The plot of these phasor quantities shows the phase and the magnitude as the distance and angle from the origin. The Nyquist stability criterion provides a simple test for stability of a closed-loop control system by examining the open-loop system's Nyquist plot (i.e. the same system including the designed controller, although without closing the feedback loop). In the present invention, the four solutions are plotted on four Nyquist diagrams in order that the optimal solution can be readily determined. Figures 4a - 4d show an example of four such control solutions.
In Figures 4a and 4d the control solutions are not stable because the route of the plot encircles the stability point -1,0. Figure 4b shows a Nyquist diagram of the first stable control solution based on remote feedback signals. The black point 14 near the real axis represents the gain stability margin and the black point 16 on the unit circle indicates the phase stability margin. The route of the plot forms a clear loop which shows that the control system will have a relatively high stability margin. Figure 4c shows a Nyquist diagram of the second stable control solution of the example in Figures 3a and 3b. The black point 18 near the real axis represents the gain stability margin. The phase stability margin is infinite in this case, as there is no intersection with unit circle. The route of the plot forms a clear loop which shows that the control system will also have a high stability margin. The dot-dash line around zero represents the unit circle.
The Nyquist diagrams for the four solutions are compared in order to determine the single solution having the highest stability for the control system. It should be noted that all four solutions are compensating the same mode and they are designed to achieve the same eigenvalue/pole shift of the critical oscillatory mode in the s-plane. However, due to the eigendynamics of the controller, each resulting closed-loop solution has totally different properties which are visible in the Nyquist diagrams shown in Figures 4a -4d. Thus, the influence on the closed loop system behaviour can be different for each solution and it may be possible to clearly identify the single solution having the highest stability for the control system. However, if none of the solutions can clearly be identified as the best solution utilising the Nyquist diagrams then a second stage in the analysis is pursued.
In this second stage, the Bode diagram of each of the solutions is constructed. A Bode diagram is a combination of a Bode magnitude plot above a Bode phase plot. A Bode magnitude plot is a graph of log magnitude versus frequency, plotted with a log-frequency axis, to show the transfer function or frequency response of a linear, time-invariant system. The magnitude axis of the Bode plot is usually expressed as decibels, that is, 20 times the common logarithm of the amplitude gain. With the magnitude gain being logarithmic, Bode plots make multiplication of magnitudes a simple matter of adding distances on the graph (in decibels), since log (a . b) = log (a) + (b). A Bode phase plot is a graph of phase versus frequency, also plotted on a log-frequency axis, usually used in conjunction with the magnitude plot, to evaluate how much a frequency will be phase-shifted. For example a signal described by: Asin(ωt) may be attenuated but also phase-shifted. If the system attenuates it by a factor x and phase shifts it by -Φ the signal out of the system will be (A/x) sin(ωt - Φ). The phase shift Φ is generally a function of frequency. Phase can also be added directly from the graphical values, a fact that is mathematically clear when phase is seen as the imaginary part of the complex logarithm of a complex gain.
Thus, Bode diagrams for the four solutions are shown in Figures 5a - 5d and are compared in order to determine the single solution having the most preferable gain characteristics. Figure 5a shows a Bode diagram of the first control solution based on remote feedback signals. Decaying gain at high frequencies can be observed. Figure 5b shows a Bode diagram of the second control solution based on remote feedback signals and high gain at high frequencies can be observed. Thus, the influence on the closed loop system behaviour caused by measurement noise and/or interaction with other modes will be different for each solution and it may be possible to clearly identify the single solution having the most preferable gain characteristics. However, if none of the solutions can clearly be identified as the best solution utilising the Bode diagrams of the designed controllers then a third stage in the analysis is pursued.
In the third stage, the complex frequency domain graph of the control solutions may be constructed. In such a complex frequency domain graph, the x-axis represents the real part of s, which is absolute modal damping, and the y-axis represents the imaginary part of s, which is modal frequency in radians per second. The s-plane transforms are commonly known as Laplace transforms hence in the s-plane, multiplying by s has the effect of differentiating in the corresponding real time domain and dividing by s has the effect of integrating. Each point on the s-plane represents an eigenvalue or a transfer function pole.
With reference to Figure 1 and 3a, a control solution is illustrated. The cross denoted as λ_k represents the situation without any damping controller and the cross denoted as λ_k,des shows an improvement in damping caused by the selected controller, because the change of the eigenvalue location is towards the left half of the s-plane.
It will be clear to the skilled person that in the majority of cases, the first stage of the analysis in which the four solutions are plotted on four Nyquist diagrams will be adequate to distinguish which is the optimal solution. In such instances, the second and third stages are not performed. However, if the comparison of the Nyquist diagrams does not reveal a single optimal solution, then the second stage can be pursued. For example, if three out of the four solutions show equally acceptable solutions, then Bode diagrams of the obtained controllers for only those three solutions are constructed and analysed. Further, if the comparison of the Bode diagrams does not reveal a single optimal solution, then the third stage can be pursued. For example, if two out of the three compared solutions show equally acceptable solutions, then complex frequency domain graphs of only those two solutions in s-plane are constructed and the location of eigenvalues analysed. This enables the single best solution to be determined.
Once the single best solution for the compensation angle has been determined, the phase shift (representative of the time delay) can be rectified. As a result, the closed loop control provides similar performance to a system in which no time delays are present in the feedback loop.
In summary, when in operation, the controller performs the method steps set out in Figure 6. In a first step 20, four parameters are obtained; the frequency of the oscillatory mode to be damped ω_k, phase shift caused by the prefilters φ_F, the phase shift caused by the residue angle φ_R, and the time-delay in the control loop Td. In a second step 22, the total compensation angle φ_c considering the effect caused by the time-delay is calculated in the following manner; $ϕ_{Td} = rem (ω_{k} . Td, 2 π)$
$ϕ = ϕ_{F} + ϕ_{R} - ϕ_{Td}$
$ϕ_{c} = rem (ϕ, 2 π)$

where rem (x, y) is the reminder after division x/y.
In a third step 24, four possible compensation angles are calculated in the presented controller design procedure (leading and lagging solutions with respect to both positive and negative feedbacks denoted as solutions A, B, C and D). The fourth step 26 of the flow diagram, shows that the four potential POD controllers are designed from the four compensation angles using the lead-lag approach phasor POD or other. In a fifth step 28, the closed loop stability and the stability margin are evaluated for each of the four solutions. The controller(s) having the highest stability margin are selected by using, for example, Nyquist diagrams. In the sixth step 30, this selection may be combined with the evaluation of the dynamic behaviour of the POD controller itself. A potential controller solution with decaying gain in high frequency range (lagging) or with decaying gain in low frequency range (leading) is selected depending on its possible interactions with other modes or controllers. This is determined through creating a plot of the gain characteristics, for example, a Bode plot. In a final step 32, the potential controller solution with the highest stability margin is selected.
The original input data for this method is obtained through repeated analysis of a power system from measured data over a predetermined period of time (a model is created from this data) or from an existing power system model and the process described in the flow diagram of Figure 6 is executed upon this model. Namely, the first action to be executed on the model comprises obtaining the parameters ω_k, φ_F, φ_R, and Td.
At the end of the procedure of the present invention the optimal compensation angle is selected and this optimal compensation angle is applied to the feedback signals through adjusting the parameters of the lead-lag controller.
In a preferred embodiment, the controller of the present invention may be run on a wide-area monitoring and control platform. In a further preferred embodiment, the controller of the present invention may be run on a PMU.
In a further preferred embodiment, the controller of the present invention may be run on a FACTS device, specifically the low level power electronics control platform for the FACTS device, or alternatively on a fast acting device such as an AVR or a direct load modulator.
It will be apparent to the skilled man that the controller of the present invention may be hardwired or implemented as a computer program.
Figure 7 schematically illustrates a power system control environment integrating a global clock. In the following way it is possible to estimate the total time delay in run time. A plurality of PMUs, PMU 1 to PMU n, and a control system are all equipped with a transceiving means 34. Further, each PMU has a standard coupling to a phasor data concentration PDC platform 36. In turn, the PDC is coupled to the control system 38. A satellite 40 provides the GPS synchronized clock system to synchronize the PMUs and the control system 38. Through provision of a global clock on the hardware platform, the time delay is determined on a continuous basis. In a first embodiment, time-stamped phasor data is recorded at regular time intervals at the remote PMUs and transmitted to the control system 38. The time delay of the remote feedback signal is determined on each occasion that the time-stamped phasor data is received by the control system 38. In a second embodiment, time-stamped phasor data is recorded continuously at the remote PMUs and transmitted to the control system. The time-stamped phasor data received by the control system is filtered and time delay of the remote feedback signal is determined as a running mean value.
Figure 8 shows a preferred embodiment of the control system 38 for a controllable device in a power system. It consists of two feedback loops for power oscillation damping (POD), which is the same as damping of electromechanical oscillations. The feedback loop on the top corresponds to a standard configuration, where the input signal is a locally measured quantity e.g. power flow on a local transmission line or locally derived frequency. At the bottom, the feedback POD loop according to the invention is indicated. It receives synchronized and, at the time of measurement, time stamped phasors 42 from the phasor data concentrator PDC. The phasors are time stamped 44 again at the time of arrival. The age of the most recently received phasor is estimated and a moving average of the time delay is estimated 46. The phasor and the moving average of the time delay are transmitted to the POD 48 such that the appropriate control signal is established. However, if the age of the most recently received phasor is too old, a switch-over to the conventional POD loop based on local measurements is effected.
In summary, the size of the time delay as determined by the control system results in one of the following outcomes:

A time delay of about 10% or less of the oscillating signal period means that the control system proceeds with the control algorithm as if there was no time delay.
A substantial time delay, but of less than 100% of the oscillation signal period, means that the control system proceeds with the control algorithm compensates for the time delay.
A time delay of 100% or more of the oscillation signal period results in the cancellation of the control algorithm to ensure that adverse effects on the power system are avoided.

Importantly, the control system initially determines 50 whether the remote measurement is to be used for control or only the standard local POD setup 52.
Further, the control system of the present invention may intentionally delay the measurement to a predetermined larger time delay.
The skilled man will be aware that such time-stamped phasor data and the associated calculated compensated controller parameters may be stored in a memory of the controller. When the actual time delay is determined by controller, then it is possible that the associated compensated controller parameters have already been calculated and need only be retrieved from the memory, thereby minimizing the processing in the controller.
Whilst the foregoing description of the invention describes a system for compensation of a time delay in the field of POD control, the skilled person will be aware that further embodiments may be envisaged. Specifically, control schemes for remote voltage control and/or control schemes for avoiding loss of synchronism.

Claims

A method for compensation of a time delay in remote feedback signals in power system control, comprising;
- determining a time delay of the remote feedback signals,

- converting the time delay into a phase shift,

- calculating (24) four compensation angles from the phase shift,

- constructing a Nyquist diagram of each compensation angle,

- determining (28) a preferred compensation angle through analysis of the four Nyquist diagrams, and

- applying the preferred compensation angle to the remote feedback signals.
The method according to claim 1, wherein the method further comprises;
- constructing a Bode diagram of at least two of the four compensation angles,

- determining (30) a preferred compensation angle through analysis of the Bode diagrams, specifically, evaluating decay of gain at higher frequencies.
The method according to claim 1 or claim 2, wherein the method further comprises;
- constructing a complex frequency domain diagram of at least two of the four compensation angles,

- determining a preferred compensation angle through analysis of the complex frequency domain diagram, specifically, evaluating eigenvalue shift with respect to other system eigenvalues.
The method according to any preceding claim, wherein the step of converting the time delay into a phase shift occurs at the dominant frequency.
The method according to any preceding claim, wherein the four compensation angles (24) are a lead compensation to +1 and a lag compensation to -1 and a lead compensation to -1 and a lag compensation to +1 of the phase shift signal.
A controller (38) for a power system performing the method for compensation of a time delay in remote feedback signals of any preceding claim.
A computer program for compensation of a time delay in remote feedback signals in power system control, which computer program is loadable into an internal memory of a digital computer and comprises computer program code means to make, when said program is loaded in said internal memory, the computer execute the functions of the controller according to claim 6.
An electric power control system, the system comprising;
at least one phasor measurement unit (PMU) equipped with transceiving means (34),
a controller equipped with transceiving means (35), and
a global clock (43) for synchronising the controller (38) with the at least one phasor measurement unit utilising the transceiving means, wherein the global clock is arranged such that a time delay of the system is determined on a continuous basis.
The electric power control system according to claim 8, wherein the time delay is estimated and the estimated time delay is utilised for phase compensation.
The electric power control system according to claim 9, wherein at least one parameter of the controller (38) is adapted on-line based on the estimated time delay.
The electric power control system according to claim 8, wherein a running mean value of the time delay is determined and utilised for phase compensation.
The electric power control system according to claim 8, wherein the at least one PMU supports a remote control loop which is inactivated if the time delay is too large for the purpose of the control.